1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to handover in a wireless local area network (LAN), and more particularly, to fast and efficient handover at a link layer of a wireless LAN based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard.
2. Description of the Related Art
In recent years, a remarkable increase in the number of users who use mobile communication services and mobile communication services that support multimedia communications has triggered a need to provide seamless communication services to moving users. Accordingly, a lot of attention has been devoted to a fast handover in a link layer of a wireless LAN based on the IEEE 802.11 standard. The handover at the link layer of the wireless LAN is a technology whereby a mobile station that moves between basic service sets (BSSs) that a plurality of access points respectively manage can detect a new access point and be associated with the new access point before it is completely disconnected from a BSS managed by an access point, thus maintaining seamless communication services. The “BSS”, which is a term used in the IEEE 802.11 standard, indicates a subnet managed by an access point.
FIGS. 1 and 2 are views illustrating related art wireless LANs. Referring to FIGS. 1 and 2, each related art wireless LAN includes a mobile station (MS) 10 and first and second access points (AP 1 and AP 2) 201 and 202. Referring to FIG. 1, the mobile station 10 is moving from a BSS managed by the first access point 201 to a BSS managed by the second access point 202. In this case, the mobile station 10 measures the strength of a signal received from the first access point 201, and scans channels available to the mobile station 10 to detect a new access point when the measured signal strength is less than a predetermined threshold. Next, the mobile station 10 measures the strength of a signal received from the second access point 202 that is detected according to the scanning result, and determines whether to perform a handover from the first access point 201 to the second access point 202, based on the measuring result.
According to the IEEE 802.11 standard, a scanning manner in which a mobile station spontaneously transmits a probe request frame and receives a response frame to the probe request frame to detect an access point is referred to as “active scanning”, and a scanning manner in which the mobile station passively receives a beacon frame to detect an access point is referred to as “passive scanning”.
Referring to FIG. 2, the mobile station leaves a BSS managed by the first access point 201, and enters a BSS managed by the second access point 202. In this case, the mobile station 10 measures the strengths of signals received from the first and second access points 201 and 202, and performs a handover to the second access point 202 when the measuring result shows that the strength of the signal received form the second access point 202 is stronger than that of the signal received from the first access point 201.
FIG. 3 is a graph illustrating the strengths of signals received from the first and second access points 201 and 202 of FIG. 1 or 2. Referring to FIG. 3, the y-axis of the graph denotes the strengths of the signals received from the access points 201 and 202, and the x-axis of the graph denotes the distance between the first and second access points 201 and 202. Referring to FIG. 3, the strengths of the signals received from the first and second access points 201 and 202 are the same at a T point on the x-axis. If the mobile station 10 moves to the right of the T point, the strength of the signal received from the second access point 202 is greater than that of the signal received from the first access point 201, and thus, the mobile station 10 performs a handover to the second access point 202.
However, as described above, conventionally, the mobile station 10 has to scan all of a plurality of channels available to detect a new access point, thereby increasing the handover latency. In particular, in the case of passive scanning, the handover must be delayed until the mobile station 10 receives a beacon frame, thereby increasing the handover latency more and more.
Also, since it is impossible to smoothly provide communication services with only an access point in a region crowded with wireless LAN users, several access points are further installed to disperse the load of communications. However, as described above, conventionally, the mobile station 10 determines whether to perform a handover only in consideration of the strength of a signal received from an access point, and therefore, the handover is concentrated to an access point that transmits a strongest signal.